Print output devices, such as copiers, printers, and printing presses, typically operate in a binary mode (i.e., a printer typically either deposits colorant or ink on a substrate or does not deposit colorant or ink on the substrate at a specific location). As a result, most print output devices are unable to directly reproduce the variety of intensity levels present in a continuous tone (“contone”) image. Instead, halftoning techniques are used to render intensity or lightness levels by converting a contone image to a halftone image. A halftone representation is an approximation of a contone image that uses a series of carefully placed spots of various sizes and/or patterns that, when viewed at a distance, creates an illusion of continuous tones.
Modern printing systems have increasingly incorporated digital image processing technology, in which an image is represented and processed in digital form prior to printing or platesetting. Such image processing techniques include digital halftoning. A common halftoning technique is screening, in which a halftone screen is created that includes a two-dimensional array of threshold values. For color images that have been separated into component colors (e.g., Cyan, Magenta, Yellow and Black), a halftone screen is created for each separation. To create a halftone output, the contone image data values are compared to the halftone screen values on a pixel-by-pixel basis for each color separation.
Holladay U.S. Pat. No. 4,149,194, describes techniques for producing halftone screens with minimum memory requirements. The size of such halftone screens typically are small compared to the image to be printed, and the screens are repeated across the entire image in a manner similar to tiling. If the contone image value for a specific pixel exceeds the corresponding threshold value, a binary “1” is specified at the pixel location. If, however, the contone image value is less than or equal to the corresponding threshold value, a binary “0” value is specified. To produce a printed output from halftone output data, a color spot is printed at the corresponding pixel locations having values of 1, and no spot is printed at corresponding pixel locations having values of 0. This process is repeated for each separation.
A common problem that arises in digital color halftoning is moiré patterns, which are visibly apparent interference patterns that result from interference between two or more halftone screens, between halftone screen dots and portions of the image, and between halftone screen dots and the pixel grid. One commonly known technique for reducing or eliminating moiré patterns involves rotating each halftone screen at a corresponding angle relative to a point of origin. For example, in a three-color process, moiré patterns may be substantially eliminated by using three halftone screens that are square in shape and identical, and placed at 15°, 45° and 75°, respectively, from a point of origin. However, for printing processes that use additional colors, or have non-square-shaped halftone screens, the process of obtaining satisfactory halftone screen values often requires an iterative modification of screen parameters.
Indeed, in traditional printing processes, once a set of screen parameters are specified, halftone threshold values are calculated, the contone data are halftoned, and the resulting output data are sent to a printer to obtain a “hard proof” of the image. In particular, proofs are printed on paper or other print media and inspected to ensure that the images and colors look visually correct. If the resulting proof includes undesirable image artifacts such as moiré, one or more screen parameters can be adjusted and successive hard copy prints can be examined in the hard proofing process. After determining that a particular proof is acceptable, the screen parameters used to make the acceptable proof can be reused to mass-produce, e.g., on a printing press, large quantities of print media that look visually equivalent to the acceptable proof. Such conventional hard proofing techniques are very time and material-intensive and expensive.
“Soft proofing” is an alternative previously known technique that can be used to adjust image processing parameters, such as halftone screen parameters, prior to production printing. In particular, soft proofing is a process that makes use of a display device rather than a printed hard copy. Soft proofing is highly desirable for many reasons. For instance, soft proofing can remove the need to print copies of the image on media during the proofing process. Moreover, soft proofing may allow many people to proof color images from remote locations simply by looking at display devices, rather than awaiting delivery of hard copies. Soft proofing can be faster and more convenient than hard proofing. Moreover, soft proofing can reduce the cost of the proofing process. For these and other reasons, soft proofing is a preferred soft proofing method for modern printing workflows.
Referring to FIG. 1, a previously known soft proofing system is described. Soft proofing system 10 includes memory 12, processor 14 and viewer 16. Memory 12 may be a hard drive, floppy disk, optical disk, or other similar memory device, and may include an image file 18 that includes contone image data separated into Cyan, Magenta, Yellow and Black (“CMYK”) colorants. Processor 14 may be a general purpose computer, such as a personal computer or workstation, that may be programmed to determine halftone screen threshold values based on user-input 18, and to apply the threshold values to image file 18 to provide halftoned data suitable for display on viewer 16.
Such previously known soft proofing processes, however, are extremely slow when used to iteratively modify and display halftone image data. For example, if image file 18 includes an 8″×10″ image at a resolution of 2400 dots/inch, the file includes more than 460 million pixels for each color separation. As a result, if the user changes screen parameters of any halftone screen, processor 14 typically will recalculate all threshold values for that screen, and then reprocess all image pixels, one pixel at a time, for the corresponding color separation to provide updated halftone data values for display on viewer 16. Such recalculation and processing may take a considerable amount of time, which makes soft proofing system 10 cumbersome and slow for purposes of iteratively determining and displaying halftone screen parameters. As a result, previously known soft proofing systems for halftoning have had limited commercial success.
In view of the foregoing, it would be desirable to provide soft proofing methods and apparatus that reduce the time required to display halftone image data.
It also would be desirable to provide soft proofing methods and apparatus that allow halftone image data to be quickly modified and displayed.
It further would be desirable to provide soft proofing methods and apparatus that allow halftone screening substantially in real-time.
It additionally would be desirable to provide soft proofing methods and apparatus that allow automatic detection and elimination of moiré.
It also would be desirable to provide soft proofing methods and apparatus that show realistic dot gain through simulated dot growth